Exposure control circuitry compensated for temperature and voltage fluctuations

ABSTRACT

In an electrical exposure control device for a single lens reflex camera in which the scene light of objects to be photographed which passes through the objective lens of the single lens reflex camera is received by light receiving elements the output thereof is detected by the compression circuit as a voltage proportional to the value of logarithm of the brightness of said light, said potential is stored in the storage means, the timing circuit is actuated, at the time of exposure, by the current anti-logarithmically converted from the stored voltage, and the activation circuit for terminating the exposure is included; there is disclosed that said device comprises a constant current generating circuit for preventing variation of current resulting from fluctuation of voltage in the power source, said voltage being impressed to the compression circuit; a circuit to which the current generated at the constant current generating circuit is fed, said circuit including photoconductive elements and resistors, and generating a voltage proportional to a value of logarithm of illumination of light receiving surfaces; a compression circuit connected in series to the secondlymentioned circuit and including a bias voltage generating circuit depending upon a set value of film speed and a set value of diaphragm aperture, said compression circuit photographically calculating voltage of both circuits which is developed by output current of said constant current generating circuit and producing output voltage proportional to a value of logarithm of duration of exposure; a storage means for storing output voltage of said compression circuit through a switch prior to exposure; a timing circuit for producing an anti-logarithmically converted current from the stored voltage of said storage means simultaneously with the exposure, the thus produced current being impressed thereto; and an activation circuit being actuated by said timing circuit and terminating the exposure, said device being free from error which may be introduced due to a variation of characteristics of P-N functions in these circuits depending upon a variation of temperature, and part of these circuits being of an integrated circuit.

United States Patent [1 1 Yata et al.

[ Dec. 11, 1973 EXPOSURE CONTROL CIRCUETRY COMPENSATED FOR TEMPERATUREAND VOLTAGE FLUCTUATIONS [75] Inventors: Kotaro Yata, lkeda; YasuhiroNanha; Masayoshi Sahara, both of Sakai, all of Japan [73] Assignee:Minolta Camera Kabushiki Kaisha,

Osaka-shi, Osaka-fu, Japan 22 Filed: Oct. 30, 1972 21 Appl. No.: 301,842

[30] Foreign Application Priority Data Nov. 2, 1971 Japan 46/86739 [52]US. Cl. 95/10 CT [51] int. Cl. G03!) 7/08 [58] Field of Search 95/10 CT[56] References Cited UNITED STATES PATENTS 3,641,890 2/1972 Ono .j.95/10 3,698,302 10/1972 Sato 95/10 3,712,194 1/1973 Yoshimura 95/10Primary ExaminerSamuel S. Matthews Assistant Examiner-Michael L. GellnerAttorneyWatson, Cole, Grindle & Watson [5 7] ABSTRACT In an electricalexposure control device for a single lens reflex camera in which thescene light of objects to be photographed which passes through theobjective lens of the single lens reflex camera is received by lightreceiving elements the output thereof is detected by the compressioncircuit as a voltage proportional to the value of logarithm of thebrightness of said light, said potential is stored in the storage means,the timing circuit is actuated, at the time of exposure, by the currentanti-logarithmically converted from the stored voltage, and theactivation circuit for terminating the exposure is included; there isdisclosed that said device comprises a constant current generatingcircuit for preventing variation of current resulting from fluctuationof voltage in the power source, said voltage being impressed to thecompression circuit; a circuit to which the current generated at theconstant current generating circuit is fed, said circuit includingphotoconductive elements and resistors, and generating a voltageproportional to a value of logarithm of illumination of light receivingsurfaces; a compression circuit connected in series to thesecondly-mentioned circuit and including a bias voltage generatingcircuit depending upon a set value of film speed and a set value ofdiaphragm aperture, said compression circuit photographicallycalculating voltage of both circuits which is developed by outputcurrent of said constant current generating circuit and producing outputvoltage proportional to a value of logarithm of duration of exposure; astorage means for storing output voltage of said compression circuitthrough a switch prior to exposure; a timing circuit for producing anantilogarithmically converted current from the stored voltage of saidstorage means simultaneously with the exposure, the thus producedcurrent being impressed thereto; and an activation circuit beingactuated by said timing circuit and terminating the exposure, saiddevice being free from error which may be introduced due to a variationof characteristics of P-N functions in these circuits depending upon avariation of temperature, and part of these circuits being of anintegrated circuit.

7 Claims, 13 Drawing Figures PAIENIEDBECHIBH 3.777,

SHEEI 1 OF 6 M I P Me 3 x a METER gag? COMPRESSION STORAGE GENERATINGCIRCUIT E] cmcun. CIRCUIT MEANS A De D Tc k V x ACTIVATION DELAY LEVELTIMING DETECTION CIRCUIT CIRCUIT CIRCUIT CIRCUIT OUTPUT CURRENTTEMPERATURE PATENIEU I I973 SIIEET 2 I 6 Log LUX COMBINATION ADJUSTMENTSOF FILM SPEED AND DIAPHRAGM APERTURE BRIGHTNESS OF AN OBJECT F I G. 4

COMBINATION ADJUSTMENTS OF FILM $3550 AN D DIAPHRAGM APERTURE SEC usE.wmnmomxm OUTPUT VOLTAGE OF COMPRESSION CIRCUIT P PATENTEBBEBI 1 msSHEEIQUFS way mum QI PAIENTED 9E9! 1 I975 3. T 7 7. 6 3 8 SHEET 6 0F 6 Ng, a O E g E I- Q 2% TEMPERATURE j z TENDENCY TO LOWER a E U i- VBEQIZVOLTAGE ACROSS THE BASE AND EMITTER OF TRANSISTOR Q12 1 2 ----soo I000Met INTEGRATED TRANSISTOR AMPLIFICATION FACTOR OF EXPOSURE CONTROLCIRCUITRY COMPENSATED FOR TEMPERATURE AND VOLTAGE FLUCTUATIONSBACKGROUND OF THE INVENTION The present invention relates to anelectrical exposure control device for a single lens reflex camera and,more particularly, to an electrical exposure control device for a singlelens reflex camera in which scene light entering an objective lens of'asingle lens reflex camera is received by light receiving elements so asto produce and store a value signal representative of the objectbrightness to be photographed. Exposure is controlled according to thestored-signal as well as by a shutter speed manually set according tothe brightness of the object to be photographed, the film speed, and thediaphragm aperture.

In the single lens reflex camera, it is well known that where theexposure is controlled by measurement of the scene light passing throughthe'objective lens, the release operation causes the camera to beshifted from the viewing condition to the exposure condition. Also thereflex mirror is shifted from the viewing position to the exposureposition. Therefore, the brightness of the object to be photographed,prior to causing the diaphragm aperture to enter a preset aperturesetting, is detected as a voltage signal by a detection circuit, thedetected voltage is stored by a storage means; a timing circuit isactuated by the stored voltage simultaneously with the start ofexposure; and when the level detection circuit detects that the voltagelevel of the timing circuit has reached a predetermined level, theoperating circuit for terminating exposure is actuated.

It is also known that the brightness of the object to be photographedwhich is detected by the detection circuit is logarithmicallycompressed; the logarithmically compressed voltage is stored in thestorage means. Thereafter the stored voltage is anti-logarithmicallyconverted. By such an arrangement, the capacity of the capacitor of thestorage means can be lessened and the voltage of the power source can belowered.

However, it is apparent that in the processes of logarithmicalcompression to the anti-logarithmical conversion, even a small errorintroduced in the logarithmically compressed stored voltage would resultin a large error of the duration of exposure to be controlled, when thestored voltage is logarithmically extended. It will be also understoodthat an error in the exposoure duration to be controlled, which isinversely proportional to the logarithmically extended storing voltage,becomes large, particularly when the birghtness of the object to bephotographed is high, in other words, when a fast shutter speed iscontrolled.

For reducing such an error to a minimum, the elimination of the errorbased upon fluctuation of the power source voltage must first be takeninto consideration. For this purpose, the voltage compression circuitmust be actuated by the output of the power source. Additionally, P-Njunctions of transistors and diodes in either the constant currentgenerating circuit, the compression circuit and the timing circuit areused for compensation. The P-N junction has a well-known characteristicwhich varies depending upon the temperature. Accordingly, considerationfor compensation of error caused due to the temperature change of theP-N junctions of the respective semiconductor elements is required.

In the actual application of the exposure control according to thestorage system in a sngle lens reflex camera, besides the value ofbrightness of the object to be photographed, values of film speed anddiaphragm aperture are employed to control the exposure time. Thediaphragm aperture, when the camera is in the viewing condition, is leftfully open corresponding to the light projected through the objectivelens, and is set to a preset value when the camera is changed into theexposure condition. Accordingly, the value of the diaphragm aperture isnormally a preset value.

Either a value of film sensitivity or a preset value of the aperturechanges such that the former is ASA 25, 50, and the latter is 1.4, 2,2.8, 4, etc., respectively, and these values must be fed as inputs tothe compression circuit.

OBJECTS OF THE INVENTION An object of the present invention is toprovide an electrical exposure control device for a single lens reflexcamera, wherein, in a system for controlling an exposure in which adetected value of illumination of an object to be photographed islogarithmically compressed, that value is stored in a storage meansprior to exposure, and at the time of exposure, the exposure iscontrolled according to the stored value, introduction of errorsresulting from fluctuation of the power source voltage and temperaturechanges are prevented.

Another object of the present invention is to provide an electricalexposure control device for a single lens reflex camera, in which, in anexposure control circuit of the type specified, and wherein a camera isprovided with a curtain shutter consisting of two curtains, there isprovided a delay circuit for compensating for exposure error resultingfrom an overlap between the trailing edge of the forward curtain and aleading edge of the rear curtain, which overlap exists when the curtainshutter is in a cocked position.

A still further object of the present invention is to provide anelectrical exposure control device for a single lens reflex camera,wherein the exposure control circuit is of a monolithic integratedcircuit construction to provide miniaturization.

SUMMARY OF THE INVENTION The foregoing objects of the present inventionare obtained by a special design of a constant current generatingcircuit, which provides a source of current to the voltage compressioncircuit for determining a voltage in accordance with the objectbrightness; and a special design of the compression and exposure timecontrol circuits. The constant current ga'nerating circuit usessemiconductor elements, such a transistors and diodes which are selectedto have matching temperature characteristics. The semiconductor elementsare connected to provide compensation for both the fluctuation of thebattery power source and temperature changes. The voltage compressioncircuit includes compound photoconductive elements for producing avoltage proportional to the logarithm of the illumination on the lightreceiving surfaces of the compound photoconductive elements. A voltagebias circuit generates a voltage in accordance with the film speed andthe diaphragm aperture setting. Both the logarithmic voltage and thevoltage of the bias circuit are combined using the output of theconstant current generating circuit to produce an output voltagerepresentative of the exposure time.'That output voltage is stored andfed to a timing circuit wherein the logarithmically compressed signal,is anti-logarithmically converted upon the initiation of exposure toactuate a circuit for terminating exposure. The voltage compressioncircuit and the timing circuit each include semiconductor elementsselected and interconnected to compensate for errors resulting fromtemperature changes.

The circuit components are intended to be constructed as integratedcircuits in order to provide miniaturized components for the exposurecontrol circuits in a camera of the type specified herein.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1- is a block diagramrepresentation of an embodiment according to the present invention;

FIG. 2 is a graph showing a temperature characteristic of the outputcurrent of the constant current generating circuit shown in FIG. 1;

FIG. 3 is a graph showing the relationship of the output voltageproportional to the logarithm value of brightness of an objectto bephotographed in the compressioncircuit of FIG. 1, in'relation torespective set values of film speed and diaphragm aperture;

.FIG. 4 is a graph showing the relation between the output voltage ofthe compression circuit and the logarithmic exposure time as a functionof different film speed and diaphragm aperture settings;

FIGS. 5(A) and 5(8) are the electric circuit diagrams of an embodimentaccording to the present invention;

FIG. 6(A) is a partial circuit diagram showing an ex emplary embodimentof a compound light receiving element for the compression circuit in theembodiment of FIGS. 5(A), 5(B);

FIG. 6(B) is a graph showing the relation between the logarithmic lightvalue on receiving surfaces of the light receiving element of FIG. 6(A),and the value of combined resistance;

FIG. 7 is a graph showing the relation between the terminal voltage ofvariable bias resistor Rs for setting values of film speed and diaphragmaperture and the values of diaphragm aperture and film sensitivity inthe embodiment of FIGS. 5(A), 5(B);

FIG. 8 is a graph showing the relationships, for different temperatures,between an output voltage of the compression circuit and logarithmiclight value of the light on the receiving surfaces of the lightreceiving elements;

FIG. 9 is a graph showing the relation between the base-emitter voltageof an input transistor of the timing circuit and the logarithmiccollector current of the same transistor;

FIG. 10 illustrates a permissible range of automatic controlasfindicated by a meter circuit; and

FIG. 11 is a graph showing the relation between the amplification of anNPN transistor and a PNP transistor and the ambient temperature.

DETAILED DESCRIPTION OF THE INVENTION In FIG. l constant currentgenerating circuit I generates a stable output current even in the eventof fluctuations in the supply voltage, while the output current thereofchanges in proportion to an absolute temperature. The characteristics ofthe output current and the termperature are shown in FIG. 2. Constantcurrent generating circuit I serves as a power source for meter circuitM and compression circuit P. Compression circuit P converts aquantitative change of incident light into a representative voltagesignal. Generally, brightness of an object to be photographed and theaperture and film sensitivity settings, all of which are taken asinputs, change as, for example, I lux, 2, 4, lux in the value ofbrightness of the object to be photographed; 1.4, 2, 2.8, 4 in theaperture settings, and 25, 50, (ASA values) of film sensitivity,respectively. Accordingly, if such changing values are used, as theyare, as outputs of measured light, the measured light outputs changeaccordingly. For example, suppose that the range of duration of exposureis limited from l to H1 ,000 second and the output voltage of thecompression circuit P is 0.1v per second. Then the output voltage is100v for l/1,000 second. Therefore, a large power source voltage isrequired and the employment of miniaturized circuitry is impossible.Consequently, the practical application of such values becomesimpossible, or else requires bulky components, or produces inaccurateand unacceptable results.

In the circuitry according to the present invention, the aforementionedproblems are overcome by using a compression circuit P so designed thatan equal-differentially changing output of the measured light isobtained from the changing input data; the characteristic relatiohshipthereof being shown in FIG. 3.

With respect to the storage means Me, in the single lens reflex camera,scene light of the object to be photographed passes through aphotographing lens system, from which it is introduced to the findersystem by a reflex mirror. Light receiving elements are disposedrearwardly from the reflex mirror so as to provide the lightmeasurement. At the time of photographing, when the reflex mirror isrotated to block the path of light to the light receiving elements, theoutputs thereof fluctuate to such an extent that the output voltage ofthe compression circuit does not represent a precise value. Therefore,in such a case, the duration of exposure cannot be controlled by theoutput of the compression circuit P. For the sake of eliminating suchinfluences due to rotation of the reflex mirror or the like, there isprovided a storage means Me, in which the output voltage of thecompression circuit P is stored on a capacitor thereby using the outputvoltage of the compression circuit P during a period required to controlthe duration of exposure.

A timing circuit Tc, in case automatic exposure control is desired,controls the changing exposure times in accordance with the changingstored voltages of storage means Me, for example, by means of atransistor and capacitor timing circuitry; whereas, for manually setexposure control a circuit consisting of a fixed resistor and capacitoris actuated.

Level detection circuit D detects the level of output of the timingcircuit Tc and generates an output signal when the timing circuit outputreaches a predetermined level.

Delay circuit De may be added inthecase where there is used a focalplane shutter having a forward curtain and a rear curtain which areoverlapped to eliminate light leakage when the shutter is cocked. Delaycircuit De is actuated to electrically adjust for an error in theduration of exposure resulting from the difference in the startingpositions between the trailing edge of the forward curtain and theleading edge of the rear curtain. The delay circuit enables theadjustment between both curtains during assembly to be simplified.

Activation circuit A amplifies the output of either level detectioncircuit D or delay circuit De and controls the energization of anelectromagnet to terminate exposure.

Finally, meter circuit M is actuated by the outputs of compressioncircuit P and constant current generating circuit 1 to indicate theduration of exposure at the time of photographing. The respectivecharacteristics of storage means Me, timing circuit Tc, level detectioncircuit D, delay circuit De, and the activation circuit A are shown inFIG. 4.

The relationships of the various parameters for the respective circuitsare represented by the following formulae.

In the compression circuit P. Vp =Kl log (L K2) Wherein L is thebrightness of the object to be photographed, K 1 is a constant, K2 is aterm which is determined depending upon a combination of the filmsensitivity and aperture values, and Vp is the output of cornpressioncircuit P which is fed as an input to timing circuit Tc.

In timing circuit Tc, level detection circuit D, delay circuit De, andactivation circuit A, the formula is:

wherein, K is a constant, T is the exposure time, a is the delay ofdelay circuit De, which is a value adjustable by the camera.

As is obvious from the above formulae, if the values of K1, K2 and K3are suitably determined, L 1/T is obtained, and thus, the duration ofexposure, T, is automatically obtained as a function of the brightnessof the object to be photographed.

Referring to FIGS. 5(A), 5(B) and constant current generating circuit I,the emitter current of transistor Q6 provides a constant current tolight receiving elements Rcdsl, Rcds2 and resistors R7, R8 of thecompression circuit P; the emitter current of transistor Q1 1 provides aconstant current to variable bias resistor Rs, the resistance of whichvaries depending upon the film speed and diaphragm aperture settings ofcompression circuit P; and the emitter current of transistor Q30provides a constant current to meter circuit M.

Now with particular reference to FIG. 5(A), resistors R1 and R2 areconnected in series with each other and are connected to the collectorof NPN transistor Q1, while a terminal of resistor R1 is connectedthrough switch S1 to the positive electrode of power source E. Theemitter of NPN transistor O1 is connected to the negative electrode ofpower source E. The base of transistor Q1 is connected to the junctionof resistors R1,

NPN transistor Q2, the base of which is connected to the collector ofNPN transistor Q1, generates collector current proportional to theabsolute temperature. The emitter of transistor Q2 is connected byresistor R3 to the negative electrode of power source E and thecollector of transistor Q2 is connected to the emitter of NPN transistorQ3. The collector of NPN transistor Q3 is connected by resistor R4 andswitch S1 to the positive electrode of power source E and also isconnected to the emitter of PNP transistor Q4. The collector of PNPtransistor O4 is connected to the base of NPN transistor Q3 and to thebase of NPN transistor Q7.

The collector of transistor O7 is connected by switch S1 to the positiveelectrode of power source E and the emitter thereof is connected to thecollector of transistor Q8. The base of transistor Q8 is connected tothe base of NPN transistor Q2. The emitter of transistor Q8, which has acurrent characteristic equivalent to that of transistor Q2, is connectedby resistor R5 to the negative electrode of power source E. The emitterof transistor Q7 is also connected through series connected diodes D1and D2 to the base of PNP transistor 04.

A circuit consisting of PNP transistor Q5 and NPN transistor Q6 isconnected through resistor R6 and switch S1 to the positive electrode ofpower source E, and is connected in an equivalent manner as the circuitconsisting of PNP transistor Q4 and NPN transistor Q3. The emitter ofNPN transistor Q6 is connected to a circuit which produces an outputvoltage proportional to the logarithm of the brightness of the object tobe photographed. In a similar manner, a circuit consisting of PNPtransistor Q10 and NPN transistor Q11 is connected through resistor R12and switch S1 to the positive electrode of power source E, and isconnected in an equivalent manner as the circuit consisting of PNPtransistor Q4 and NPN transistor Q3. The emitter of NPN transistor Q11is connected to a circuit which produces a logarithmically compressedbias voltage depending upon the film speed and aperture settings.

Similarly, a circuit consisting of PNP transistor Q29 and NPN transistorQ30 is connected through resistor R39 and switch S1 to the positiveelectrode of power source E, and is connected in an equivalent manner asthe circuit consisting of PNP transistor Q4 and NPN transistor Q3. Theemitter of transistor Q30 is connected to meter circuit M. The bases ofPNP transistors Q5, Q10, Q29 are all connected to the base of PNPtransistor Q4.

Accordingly, if the voltage of power source E decreases, the operatingcurrent of transistor 01 decreases, resulting in a decrease in thebase-emitter voltage V of transistor ql. However, the voltage acrossresistor R2 also decreases and therefore the collector potential oftransistor Q2 is held stable against voltage fluctuation of power sourceE. This fact can be demonstrated by the following analysis. It isassumed that Te is the absolute temperature, Isl is the saturationcurrent of transistor Q1 at temperature Te, [s2 is the saturationcurrent of transistor Q2 (the saturation current is determined for eachtransistor and varies depending upon temperature), then the followingformula applies:

wherein, in order that the emitter current I2 of transistor Q2 is stableregardless of any fluctuation in the emitter current ll of transistorQ1, aI2/aI1 0, and hence R211 KTe/q (R2I1 z 26 mv if Te 300K) andtherefore, resistors R2, R3 must be set to satisfy the above formula.

Between the base-emitter of transistor Q1 and transistor Q2, adifference in the base-emitter voltage V BE is produced either byproviding a difference between emitter current I1 and I2, or byproviding a difference between the areas of theemitter layers. Thetemperature coefficient of the base-emitter voltage V VBE/ (KTe/q) ln(II/[B1 .1)

frornwhich. the following formula is obtained:

dVBE/dTe (VBE/Te) (KTe/q) (dlnIsl/dTe) With increases in V dVBE/dTedecreases. Accordingly, the difference in V BE between transistors Q1and Q2 varies as the temperature changes. In other words, the emittercurrent I2 of the transistor Q2 is affected by temperature change. Inthe present invention, a difference of several 10 mv exists so as to setthe collector current of transistor Q2 proportional to the absolutetemperature. If a difference in V is represented by the differencebetween emitter current I1 and I2 of transistors Q1 and Q2, and iftransistors Q1 and Q2 have equal characteristics, and the amplificationfactors of which are sufficiently large, then, the following formulaeapply:

Isl Is2' I2 (TZ/TI) I2 (wherein IsI Is2),then, the formula: lnIl ln I2[T2/(T1 T2)] In T2/Tl =(q/KT1) R312 is obtained.

In the above formulae, T1 and T2 are determined by the requiredtemperature range, and the emitter current 12 of transistor Q2 is knownsince it is a required current, as well. Then by assigning a stabilizingvalue to resistor R3, the value of emitter current II of transistor O1is obtained.

It should be noted that with respect to transistors Q3, Q4, Q5, Q6, Q10,Q11, Q29 and Q30, should an integrated cuircuit be used, theamplification factor of PNP transistors is greatly lowered to such anextent that it is impossible to use PNP transistor alone, and therefore,in the present invention, PNP transistors and NPN transistors areemployed in combination, to thereby increase the ampliflcation factor.As is apparent from FIG. (A), the approximate emitter currents oftransistors Q6, Q11 and Q30 are obtained from the following formulae,respecitvely:

Because the emitter current I2 of transistor Q2 remains stable withrespect to voltage fluctuation of power source E, and is proportional tothe absolute temperature, output currents, I6, III or I30 are alsostable with respect to voltage fluctuations of power source E and arealso proportional to the absolute temperature. A circuit consisting oftransistors Q7, Q8, diodes D1, D2 and resistor R5 generates the basecurrent of the aforesaid PNP-NPN transistor combination circuits. Inthat circuit the base curent flows to diodes D1, D2, while the currentfed to transistor Q7 and the current of transistor Q8 have a constantcurrent characteristic equivalent to the current characteristic oftransistor Q2. In the event that the amplification factors of the PNPtransistors are extremely low, the characteristics of emitter currentsI6, II I or I30 are not affected, even if the base current is large.

With continuing reference to FIG. 5(A), compression circuit P is acircuit for converting analogicallychanging input photographic data,such as the brightness of the object to be photographed, and apertureand film sensitivity settings and the like, into anarithmetically-changing output voltage.

Two photoconductive elments Rcdsl and Rcds2 are combined into aso-called composite photoconductive element, and each photoconductiveelement, as seen in FIG. 6(A), consists of a photoconductive elementcdsh of high sensitivity and a photoconductive element ads! of lowsensitivity, and are connected in parallel with each other. Highsensitivity photoconductive element cdsh is connected in series toresistor r1 and resistor r2 is connected in series with the parallelcircuit. The logarithm of the combined resistance of the aforesaidresistors is linear over a wide range relative to the logarithm of thescene light, as is clearly shown in FIG. 6(B).

Two photoconductive elements, one of which is shown in FIG. 6(A), areemployed in the circuit of FIG. 5(A), in which the high sensitivityphotoconductive elements (on the side of low resistance) are connectedwith each other through variable resistor R9 and in parallel with whichthe low sensitivity photoconductive elements (on the side of highresistance) are directly connected with each other, and variableresistor R9 is connected as shown in FIG. 5(A). The two compoundphotoconductive elements are mounted in a finder light path system (forexample, at a front edge and a rear edge surface of a roof-shaped reflexpentagonal prism). By adjusting the resistances of variable resistors R9and R9, the output voltage varies arithmetically in relation to theanalogically-changing value of brightness of the object to bephotographed which is referred to as the CLC effect.

Resistors R7 R8 connected in series with each other are connected inparallel with the above described compound photoconductive circuit.Resistors R7, R8 and the photoconductive circuit are connected to thecollector of transistor Q9, the base of which is connected to thejunction of resistors R7 and R8. Transistor Q9 provides temperaturecompensation for the base-emitter voltage of transistor Q12- Resistor Rand thermistor Rth are connected to the emitter of transistor Q9. Sinceemitter current 16 of transistor Q6, or, in other words, the operatingcurrent of transistor Q9, varies in proportion to the absolutetemperature, the base-emitter voltage of transistor Q9 also changes inaccordance with the temperature change, contrary to the case where theemitter current I6 of transistor Q6 is constant. If emitter current I6decreases as the temperature falls, the variation of the base-emittervoltage is less than the case where emitter current I6 is constant. Thatdifference is cmpensated by resistors R10 and Rth. This compensation isrepresented by the following formula:

' AVBE (KTZ/q) InI6 (KT2/q) In [16' (T2/Tl) wherein, if the variation oftemperature is 50C, the difference is approximately 4 mv, which iscompensated.

Variable resistor Rsis a means for feeding data such as the values ofaperture and film sensitivity. The voltage produced across Rs is l6-RsI6 X (nr) wherein, n is the nubmer of set levels and r is a value ofresistance per 1 electron volt (EV). By setting n according to thevalues of aperture and film sensitivity, the voltage being producedacross resistor Rs has the variation as shown in FIG. 7. Now, the sum ofoutputs Vp of the compression circuit P (the voltage between thepositive side of resistor R9 and the negative electrode of power sourceE) are represented by the formula:

wherein, R is the combined resistance of resistors R10, Rth and R11;Rcds is a combined resistance of resistors Rcdsl, Rcds2 and R9. Theseare shown in FIG. 8 for a typical example, in which Rs and Rcds, that isto say, the values of aperture, film sensitivity and brightness of theobject to be photographed are shown as a function of the temperaturechange. As is apparent from that graph, so far as the output voltagecharacteristic is concerned, since the emitter current I11 or I6 isstable in relation to the voltage of power source E, the output voltageis stabilized. Meanwhile, in relation to the temperature, the outputvoltage is shifted parallel, because the base-emitter voltage VBE oftransistor Q9 increases as the ambient temperature falls, and sinceemitter currents I11 and I6 are proportional to the absolutetemperature, the gradient of output voltage Vp becomes less.

In connection with the storage means Me, as shown in FIGS. 5(A) and5(8), wherein output voltage Vp of the compression circuit P is stored,output voltage Vp is cut off by means of switch S2 when the mirror isturned. Thus, output voltage Vp is stored for the period required tocontrol the duration of exposure and is fed as an input to timingcircuit Tc, and the change of output voltage Vp for a duration ofexposure requires a nominal value of capacity. In this invention, theduration of storage produces a negligible error of the duration ofexposure.

Timing circuit Tc, as shown in FIG. 5(B), is composed of transistor Q12controlled by switch S4 being opened in conjunction with the shutterrelease and according to an input from the storage means which storesthe output Vp of the compression circuit P. Transistor Q12 and capacitorC2 are connected so as to produce an output at the collector oftransistor Q12, whereby the duration of exposure is automaticallycontrolled by setting the values of aperture and film sensitivity.Variable resistor RT and capacitor C3 are connected in series with eachother to produce an output at their junction, so that a desired durationof exposure is manually controlled by change-over switches AMI and AM2.

Assuming a reference voltage VT of the level detection circuit D betweenthe base of transistor Q14 and thepositive electrode of power source E,the duration of exposure T is obtained by the following formula. In caseof a manual control, as is well known, the formula ISI 1 VT 1 .E

For automatic control, if the collector current of tranistor Q12 isICQ12, and if R35 0, the duration of expsoure T is: T =C2VT/ICQ12 15 Asis well known for manual control, T is stable either for a variation inthe power source E voltage or for changes in temperature, and therefore,further elaboration is not necessary. However, in the case of automaticcontrol T is a function of the voltage variations of power source E andthe changes in ambient temperature. In relation to fluctuations of powersource E voltage in formula 15, VT is given by the formulaR15/(R15-l-R16+R13)-E, in view of FIG. 5(8), and, if R13 (R15 R16), thenAccordingly, the duration of exposure T is not subject to variation inthe event of fluctuation of voltage in the power source E, and it issufficient to vary ICQ12 at a fixed rate. That is to say, it is enoughto vary the base potential of transistor Q12.

In case resistor R13 and capacitor C1 are connected as shown in FIG.5(B), ICQ12 changes from 11 to I2, when the voltage of power source Evaries from E1 to E2, the amount of fluctuation VBEQ12 of thebaseemitter voltage VBEQ12 of transistor Q12 is obtained as follows inthe generic formula of transistor:

VBGQIZ (KTe/q) In (ll/Is) (KTe/q) In (12/13) (KTe/q) In 11/12 (17) and,suppose that the duration of exposure T is constant,

accordingly, formula 16 becomes:

VBEQ12 (KTe/q) ln E1/E2 wherein, if E E1 E2, the following formula isobtained:

(wherein AE is the amount of fluctuation of voltage in the power source,and the value of resistance of the resistor R13 is set so as to satisfyformula 16.) For example, if E1 6V, and E2 2V, the voltage acrossresistor R13 is approximately 60 mv. As a result, a substantiallyperfect voltage is compensated for that fluctuation of voltage in thepower source E, which takes place when the electromagnet Mg, whichrequires a large current, is rendered conductive.

In the event of a temperature change, in order to maintain the value ofduration of exposure T free from fluctuation, collector current ICQ12 oftransistor Q12 must be kept stable. In general, the base-emitter voltageof a transistor is represented by VBEQ12 z (KTe/q) ln (ICQl2/Is) andits-characteristic is shown in FIG. 9. As is apparent from FIG. 9, thereis a parallel shift in the characteristic and a variation in itsgradient, as the temperature changes. The gradient varies in proportionto the absolute temperature, as is understood from the foregoingformula. Accordingly, so far as the duration of exposure T is-concerned,compensation is made so as to off-set these two functions of thebase-emitter voltage VBEQ12 of transistor Q12.

In view of the formula representing the output voltage Vp of thecompression circuit P, if a transistor having a characteristicequivalent to those of transistors Q9 and Q12 is employed, the variationof the baseemitter voltage VBEQ9 of transistor Q9 compensates for theparallel shift variation in the base-emitter voltage VBEQ12 oftransistor Q12. The gradient component, which varies in proportion tothe absolute temperature, is compensated by the emitter currents I11, I6which are proportional to the absolute temperature.

Particularly, in case of a short duration of exposure, the voltage oncapacitor C1 is slightly discharged through transistor Q12 and resistorR13. If the base current of transistor Q12 is ibQ12, the potential atthe base of transistor Q12 is lowered by the value ibQlZ R13. Therefore,this change in potential is added to the potential at the base oftransistor Q12 and is related to the duration of exposure. For example,1.1 mV for l/1,000S =1 mV;2.l mV for l/500S 2 mV; 4.1 mV for l/250S 4mV.

To compensate for this, if variable resistor R35 is interposed betweentransistor Q12 and capacitor C2 as shown in FIG. 5(B), then,

VT= (l/C2) lCQ12dt+ ICQ12 R35 becomes T (C2VT/1CQ12) C2R35 It will beapparent from the above formulae that a fixed value of secondsequivalent to delay elements C2R35 can be deducted. For example, if 1.1mV is to be equivalent to 1 m8, and if C2 l uF, the value of R35 is 1000.

Level detection circuit D in FIG. 5(B) is so composed that the voltagedivided by resistors R15, R16 or a variable resistor R22, is fed to oneterminal of the input of level detection circuit D, so that voltageserves as a reference voltage (which is the above voltage VT). The otherterminal of the input is connected to the output terminal of timingcircuit Tc, and voltage of capacitors C2 and C3 is compared to thatreference voltage.

In FIG. 5(B), transistors Q13 and Q14 form a known type of differentialamplification circuit, in which transistor Q16 serves as a constantcurrent supply source. A circuit consisting of transistor Q17 andresistors R17, R18 is identical in composition with the circuitconsisting of transistor Q1 and resistors R1, R2 in the constant currentgenerating circuit I. Since the collector current of transistor Q1 isstable in relation to fluctuations of voltage in the power source, thecollector current of transistor Q12 is extremely stable in relation tofluctuations of voltage in the power source. As for temperature changes,should the base of transistor Q16 be connected directly to the collectorof transistor Q17, it follows that the collector current of transistorQ16 is subject to variation, and therefore, a buffer circuit consistingof diodes D3, D4 and resistors R19, R20 are provided to stabilize thecollector current. By means of this circuit, transistors Q13 and Q14 arestably actuated to produce the stable output at the collector oftransistor Q22 which is an output of level detection circuit D.

Delay circuit De, as shown in FIG. 5(B), is used in the case wherein thecircuitry of the present invention is applied to a camera having a focalplane shutter, and a fixed time delay must be added to the timeestablished by timing circuit Tc, because of the difference in thestarting positions between the forward curtain and the rear curtain, asexplained above. In delay circuit De, variable resistor R28 andcapacitor C4 are connected in series with each other. Capacitor C4 isconnected to transistor Q23 to render it conductive or nonconductiveaccording to the output of level detection circuit D, whereby capacitorC4 is effectively shortcircuited or in an opened condition. With the aidof the charging characteristic of capacitor C4 and variable resistorR28, a fixed rate of time lag is provided. In the circuit, the junctionbetween resistor R28 and capacitor C4 serves as an output point.

Activation circuit A detects the variation of voltage in the outputdelay circuit De, and amplifies it to actuate electromagnet Mg.

In FIG. 5(B), the voltage of power source E is divided by diodes D5, D6and resistors R29, R30, to produce a reference voltage at the junctionof resistors R29 and R31). The reference voltage and variation of outputvoltage at the output of delay circuit De are compared and detected bytransistor Q24, and amplified by transistors Q25, Q26 and Q27. In thiscircuit, diodes D5 and D6 are employed to compensate in the event offluctuation of voltage in the power source E and for temperaturechanges. In this case, the formulae are:

wherein, if VDS VD6 VBEQ24', R29 R30, then t= C4R28 ln2. Thus, astabilized fixed rate of time lag is obtained. In the camera providedwith a focal plane shutter, the electromagnet Mg actuates armaturesassociated with the retention and release of the rear curtain. ResistorR31 connected in series with electromagnet Mg adjusts the current flowto electromagnet Mg to set'the most suitable value. Diodes D7, D8 and D9are arranged for the purpose of preventing transistor Q27 from beingaffected due to the counter-electromotive force of electromagnet Mg.Capacitor C is connected in parallel with electromagnet Mg and resistorR31, and compensates for the variation between intervals of actuation ofthe armatures by eletromagnet Mg. This enhances the accuracy of thecircuit performance especially when a short duration of exposure iscontrolled. Both resistors R32 and transistor Q32, the collector ofwhich is connected to one terminal of the input side of level detectioncircuit D, are provided for preventing oscillations caused by thefollowing reason.

A power source in a camera is subject to restriction in size because ofthe dimensions of the camera, and therefore, dry cells employed thereinmust be extremely small. In that case, should the large quantity ofcurrent (for example 30mA) be fed to the electromagnet Mg, it wouldcause a voltage drop of the dry cells. Further, when the electromagnetis rendered nonconductive, the load on the dry cells drasticallydecreases, as a result of which the voltage thereof instantaneouslyrises slightly, and thereafter, is returned to the original condition.Accordingly, in timing circuit Tc and level detection circuit D,assuming that the instantaneously rising voltage in the supply source isAE, then, the base voltage of transistor Q14 changes to AE[R15/(R15+R16+R13)], while the base of the transistor Q13 of thedifferential amplification circuit does not follow the variation ofvoltage in the power source B, because of capacitors C2 and C3 connectedthereto. Consequently, the base voltage of transistor Q13 varies by AEand the repetitive variation of the base voltage causes the circuit tooscillate.

Meter circuit M, as shown in FIG. 5(A), actuates meter Met by the outputof compression circuit P. Meter Met indicates a set value of duration ofexposure as shown in FIG. 10, and provides a warning, when a set valueof duration of exposure which is determined by a combination of thevalues of aperture, film sensitivity and brightness of the object to bephotographed, exceeds a permissible range. In FIG. 5(A), inputs to metercircuit M are fed to the base of transistor Q31, and if the potentialthereof varies, the emitter potential of transistor Q31, is representedby the following formula:

VEQ30 Vp 'i- VBEQ31 +I30R36 Accordingly, the emitter potential varies ina manner similar to the outupt poetential Vp of compression circuit P.The emitter potential of transistor Q34 is represented by the followingformula:

Vp VBEQ31 I30R36 VBEQ33 VBEQ34 Accordingly, the variation of the emitterpotential of transistor 034 is equivalent to that of the output voltageVp of compression circuit P. The output voltage Vp of this circuit isleft unchanged in the event of fluctuation of voltage in the powersource, and emitter current I30 of transistor Q30 is maintained stable,thereby ensuring accurate meter indication. Also, with respect tovariations of ambient temperture, an off-set to the temperture change ismade in the sum of the baseemitter voltage VBEQ9 of transistor Q9 andvoltage VBEQ31 of transistor Q31 and in the sum of the baseemittervoltage VBEQ33 of transistor Q33 and voltage VBEQ34 of transistor Q34.The emitter current I30 of transistor Q30 varies in proportion to theabsolute temperature. The temperature coefficient thereof isapproximately 3,300 PPM/C, while the temperature coefficient of theinternal resistance of meter Met is approximately 3,800 PPM/C, becauseits coils are made of a copper wire. I-lence, these temperaturecoefficients off-set one another to ensure the stable and preciseindication of meter Met. Furthermore, variable resistor R37 connected inseries with meter Met to the emitter of transistor Q34 is employed foradjusting the gradient characteristic of meter Met.

Since the emitter current 130 of transistor Q30, as set forth, varies inproportion to the absolute temperature, the operating current oftransistor Q31 also shows an equivalent variation.

In detail, the base-emitter voltage VBEQ31 of transistor Q31 varies inlike manner to that of the baseemitter voltage VBEQ9 of transistor Q9 ofthe compression circuit, to cause a slight error of meter Met. Althoughthe error is small to such extent that no special compensation isnormally required, there is provided in the present invention, forcompensation purposes, resistor R38 interposed between the output of thecompression circuit and transistor Q31. Generally, in an integratedcircuit, the amplification factor of an NPN transistor decreases byapproximately 30 percent with every 50C decrease of temperature, whilethat of a PNP transistor decreases approximately by 50 percent.Accordingly, if transistors Q31 and Q32 in meter circuit M are in anintegrated circuit, the amplification factor thereof decreasesapproximately by a total of percent. In detail, the base current oftransistor Q31 increases with a decrease in temperature, and then, thevoltage across resistor R38 varies with a change in temperature.Accordingly, if the voltage variation across resistor R38 is set to beequivalent to the variation of the base-emitter voltage VBEQ31 oftransistor Q31, the indication error of meter Met can be minimized to anegligible extent. In case transistors Q31 and Q32 are normaltransistors, the decrease in the amplification factor of either an NPNor a PNP transistor is approximately 30%, and therefore, the resistanceof resistor R38 must be set accordingly.

In the constant current generating circuit, it is obvious that avariation of resistance in resistor R6 causes a variation in emittercurrent I6 of transistor Q6. In the compression circuit P, the variationin the characteristics of two composite photoconductive elements Rcdsland RCdS2 is adjusted by variable resistors R9 and R9 so that thegradient variation of the set values of the brightness of the object tobe photographed correspond with the characteristic of input transistorQ12 of the timing circuit. Therefore, in order to change the collectorcurrent ICQ12 of transistor Q12 in terms of sets of 2, in view of thegeneric formula for a transistor the value per level must be:

VBEQlZ (KTe/q) M2 The value is approximately 17.8 mv at 25C.Accordingly, the resistance of resistor R6 must be adjusted so as to setthe value of emitter current 16 of transistor Q6 correspondingly.

- It is also apparent that a variation in resistance of resistor R12will cause a variation in the emitter current III of transistor QMJTheemitter currents l6 and III are combined and fed to variable resistorRs, by means of which data relating to the values of aperture and filmsensitivity are set. Resistor Rs, generally, is not uniform butirregular from a manufacturing standpoint. In the present invention, thevariation in the gradient component is adjustable by adjusting theemitter current III of transistor Q11. In the event that the outputvoltage changes due to a variation of resistor Rs, resistor Rs isadjusted so that the gradient component corresponds with thecharacteristic of the base-emitter voltage VBEQ12 of the inputtransistor of timing circuit Tc.

Changing the resistance value of variable resistor R11 will shift thecharacteristic of output voltage Vp of compression circuit P parallel tothe characteristic curves shown in FIG. 3. Resistor R11 is employedprimarily to adjust the variation in the base-emitter voltage VBEQ9 oftransistor 09 and voltage VBEQ12 of transistor Q12. For example,avoltage variation of approximately i 2mv is encountered in the casewherein these transistors are in an integrated circuit. Also thecapacity variation of capacitor C2 is also properly adjusted by resistorR11.

It is also obvious that if the resistance of resistor R39 varies, thenthe emitter current of transistor Q30 changes. And the change of emittercurrent I30 causes a variation in the emitter potential of transistorQ33 in the meter circuit, namely, a variation in the emitter potentialof transistor Q34. Since this means a variation in the current to meterMet, the zero point of meter Met is made adjustable by adjustment of theresistance of resistor R39.

Variable resistor R22 of level detection circuit D is provided forsetting the reference voltage thereof which is required for manuallysetting the duration of exposure. By adjusting resistor R22, a variationin the delay provided by capacitor C3 and resistor RT can becompensated.

Resistor R14 of timingcircuit Tc discharges capacitor C2 or C3. ResistorR14 serves to maintain capacitors C2 and C3 from being affected by heatresulting from the drastic discharge thereof. Normally, the impressionof an operating voltage of 3 Q-per volt will stabilize capacitors C2 andC3.

Change-over switches A-M(ll), A-M(2) and A-M(3) serve to switchdetection circuit D from manual control to automatic control, and viceversa. These switches are actuated substantially simultaneously.

Where the circuit is used for manually controlling the duration ofexposure, the respective contacts of change-over switches A-M(l-3)contact the M switch terminals in FIG. 5(B). Variable resistor RT is setto a required resistance value, and simultaneously with the release ofthe shutter, switch S5 is closed to thereby actuate timing circuit Tc,level detection circuit D, delay circuit D3 and activation circuit Ashown in FIGS. 5(A), 5(B). Then, normally closed switch S3 is opened,and when switch S4 is closed simultaneously with the start of exposure,delay capacitor C3 is charged through variable resistor RT until itsvoltage becomes equivalent to the reference voltage set by variableresistor R22, whereby level detection circuit D is actuated to renderelectromagnet Mg non-conductive through delay circuit De and activationcircuit A.

Where the duration of exposure is automatically controlled, in FIGS.5(A) and (B), the contacts of changeover switches A-M(l-3) contact the Aswitch terminals, and sliding terminal b of variable resistor RT ismoved to point a. And then, variable resistor Rs is adjusted to set avalue of film sensitivity and normally open switch S1 is closed tothereby actuate meter circuit M, constant current generating circuit I,compression circuit P, and storage means Me. And, a value of aperture isset with the aid of variable resistor Rs so as to allow meter Met toindicate a desired value of duration of exposure. By this time, switchS2 has been closed to impress on capacitor C1 the potential equivalentto that of output voltage Vp of compression circuit Thereafter,simultaneously with the start of shutter release, switch S2 is opened,before the value of'illumination on the light receiving surfaces oflight receiving elements RCdSI, RCds2 is subject to fluctuation due torotation of the mirror, whereby the voltage representing aperture, filmsensitivity and brightness of the object to be photographed are storedby capacitor C1. As

- the shutter release operation progresses, opened switch S5 is closedto actuate timing circuit Tc, level detection circuit D, delay circuitDe and activation circuit A. Briefly, electromagnet Mg is renderedoperative. Thereafter, closed switch S3 is opened, while opened switchS4 is closed simultaneously with the start of exposure, wherebycapacitor C2 starts charging through transistor (H2. The charged voltageis impressed onto the base of transistor Q13 of level detection circuitD. When the voltage becomes equivalent to the reference voltage, thevalue of which is set by resistors R15, R16 and R13 and which isimpressed on the base of transistor Q12, level detection circuit D isactuated to energize electromagnet Mg through delay circuit De andactivation circuit A, thereby terminating the exposure. In the foregoingdescription, although variable resistor R22, fixed resistors R15, R16and capacitors C2, C3 are operated by change-over switches A-M(1) and A-M(2), it will be understood that one of the two, viz. the switch A-M(1),may be omitted because capacitors C2 and C3 are operable by a singleswitch. In case the cir cuitry of the present invention is applied to asingle lens reflex camera, it is recommended that meter circuit M,constant current generating circuit 1, compression circuit P, storagemeans Me and timing circuit Tc, are mounted around the pentaprism, andto mount level detection circuit D, delay circuit De and activationcircuit C in the camera body. Such an arrangement of the circuitry inthe camera allows the finder system to be mounted in various ways. Andthe provision of switch system A-M(I), A-M(2) and A-M(3) facilitates theabove arrangement.

As will be understood from the foregoing description, the powerconsumption is greatly reduced, because of the provision of switch S5,by which the current is fed, only during exposure, to electromagnet Mg,which has the largest current consumption in the circuit. Additionally,since the conventional circuit is of the differential type,disadvantages havebeen encountered in that a large quantity of currentmust be fed to actuate the meter. Therefore, in the event of a decreasein the resistance of the photoconductive elements and the resistor whichsets values of aperture and film sensitivity, the current will increase.For example, if the brightness of the object to be photographed is high,a current of several mA must be supplied depending upon the aperturesetting. This may decrease the life of the power source.

On the contrary, according to the present invention, data such as thevalues of current flow to meter Met, the brightness of the object to bephotographed, aperture and film sensitivity settings are obtainedthrough the constant current generating circuit, and whatever thecombination thereof may be, the current consumption is decreased. It' isadvantageous that with a current of several ten ;:.A to 100 uA, thecircuit is actuated with the current consumption substantiallyequivalent to that of the conventional actionmeter.

In the foregoing description, the resistors and transistors of constantcurrent generating circuit 1, transistor Q9 of compression circuit P,transistor Q12 of timing circuit Tc are respectively constructed in amonolithic integrated circuit. It is possible, however, to form acircuit equivalent to the circuit of the present invention byconventional circuit construction techniques or by mixed integratedcircuits, and in that case, the transistors are replaceable bytransistors of a counter type, for example, a PNP transistor may replacean NPN transistor, and vice-versa.

What is claimed is:

1. An electrical exposure control device for a single lens reflexcamera, comprising:

a power source having positive and negative terminals;

a constant current generating circuit for generating currentproportional to absolute temperature and including:

a resistance consisting of a first resistor, one terminal of which isconnected to said positive terminal, and a second resistor connected inseries to said first resistor,

a first NPN transistor having a collector connected to said secondresistor, an emitter connected to said negative terminal, and a baseconnected to the junction of said first and second resistors,

second NPN transistor having a base connected to the collector of saidfirst NPN transistor and an emitter connected through a third fixedresistor to said negative terminal,

a first transistor circuit consisting of a PNP transistor having anemitter connected through a fourth resistor to said positive terminal,and a third PNP transistor having a base connected to a collector ofsaid PN P transistor, a collector connected to the emitter of said PNPtransistor and an emitter connected to the collector of said second NPNtransistor,

a second transistor circuit having a circuit connection equivalent tosaid first transistor circuit and consisting of a PNP transistor havinga base connected to the base of the PNP transistor of said firsttransistor circuit and an emitter connected'through a fifth resistor tosaid positive terminal, and an NPN transistor;

a compression circuit including light receiving elements connected to anemitter of the NPN transistor of said second transistor circuit andwhich produces a potential proportional to the logarithm of light on thereceiving surfaces of said light receiving elements;

means for storing an output of said compression circuit; and

a timing circuit including a fourth NPN transistor having a base forreceiving the voltage of said compression circuit at the time ofexposure.

2. An electrical exposure control device for a single lens reflex cameraas in claim 1, further comprising:

a third transistor circuit in said constant current generating circuitand connected equivalently to said first transistor circuit, andconsisting of a PNP transistor having a base connected to the base ofthe PNP transistor of said first transistor circuit and an emitterconnected through a sixth resistor to said positive terminal, and

an NPN transistor;

a fifth NPN transistor in said compression circuit for producing avoltage proportional to the logarithm of illumination on the lightreceiving surfaces of said light receiving elements, said fifth NPNtransistor having a similar temperature characteristic to that of saidfourth NPN transistor;

a variable resistor having one terminal connected to an emitter of saidfifth NPN transistor and connected to the emitter of the NPN transistorof the third transistor circuit of said constant current generatingcircuit and another terminal is connected to said negative terminal, andthe resistance of said variable resistor varying in accordance with thediaphragm aperture and film speed settings.

3. An electrical exposure control device for a single lens reflex cameraas in claim 1, wherein said timing circuit includes a first capacitorconnected to the collector of said fourth NPN transistor, and a secondcapacitor in parallel with said fourth NPN transistor and said firstcapacitor, said second capacitor and said series connected firstcapacitor and said fourth NPN transistor being selectively connectedthrough a first change-over switch to a second variable resistor theresistance of which varies according to the diaphragm aperture and filmspeed settings.

4. An electrical exposure control device for a single lens reflex cameraas in claim 3, further comprising:

a second change-over switch for establishing either automatic exposurecontrol or manual exposure control; I i,

a level detection circuit connected through said second change-overswitch to one terminal of either of k said first capacitor or saidsecond capacitor for detecting the voltage of either of said first andsecond capacitors, to thereby generate a signal when said voltagereaches a predetermined level; and

an electromagnet for terminating exposure and actuated by said leveldetection circuit.

5. An electrical exposure control device for a single lens reflex cameraas in claim 4, wherein said means for storing includes a capacitor forstoring the output voltage of said compression circuit and a fixedresistor connected to said capacitor and said negative terminal tothereby compensate for a voltage drop of said power source which resultsfrom the current flow to said electromagnet.

6. An electrical exposure control device for a single lens reflex cameraas in claim 5, further comprising a delay circuit connected between anoutput terminal of 19 20 said level detection circuit and an inputterminal of said tor having a base connected to the base of the PNPOperating circuit to compensate for the time interval transistor of saidfirst transistor circuit and an emit- Kepresentiflg the Overlap betweena trailing edgepf a ter connected through a resistor to said positivefirst curtain and a leading edge of a second curtain at terminal, and anNPN transistor; and

the cocked position thereof.

7. An electrical exposure control device for a single lens reflex cameraas in claim 3, further comprising: compression circuit )8 fed and toWhlCh the emitter a fourth transistor circuit connected equivalently tocurrent of 331d NPN transistor of Said fourth said first transistorcircuit in said constant current SlStOl' circuit is gen'erating circuitand consisting of a PNP transis- 10 a meter circuit to which the outputvoltage of said

1. An electrical exposure control device for a single lens reflexcamera, comprising: a power source having positive and negativeterminals; a constant current generating circuit for generating currentproportional to absolute temperature and including: a resistanceconsisting of a first resistor, one terminal of which is connected tosaid positive terminal, and a second resistor connected in series tosaid first resistor, a first NPN transistor having a collector connectedto said second resistor, an emitter connected to said negative terminal,and a base connected to the junction of said first and second resistors,a second NPN transistor having a base connected to the collector of saidfirst NPN transistor and an emitter connected through a third fixedresistor to said negative terminal, a first transistor circuitconsisting of a PNP transistor having an emitter connected through afourth resistor to said positive terminal, and a third PNP transistorhaving a base connected to a collector of said PNP transistor, acollector connected to the emitter of said PNP transistor and an emitterconnected to the collector of said second NPN transistor, a secondtransistor circuit having a circuit connection equivalent to said firsttransistor circuit and consisting of a PNP transistor having a baseconnected to the base of the PNP transistor of said first transistorcircuit and an emitter connected through a fifth resistor to saidpositive terminal, and an NPN transistor; a compression circuitincluding light receiving elements connected to an emitter of the NPNtransistor of said second transistor circuit and which produces apotential proportional to the logarithm of light on the receivingsurfaces of said light receiving elements; means for storing an outputof said compression circuit; and a timing circuit including a fourth NPNtransistor having a base for receiving the voltage of said compressioncircuit at the time of exposure.
 2. An electrical exposure controldevice for a single lens reflex camera as in claim 1, furthercomprising: a third transistor circuit in said constant currentgenerating circuit and connected equivalently to said first transistorcircuit, and consisting of a PNP transistor having a base connected tothe base of the PNP transistor of said first transistor circuit and anemitter connected through a sixth resistor to said positive terminal,and an NPN transistor; a fifth NPN transistor in said compressioncircuit for producing a voltage proportional to the logarithm ofillumination on the light receiving surfaces of said light receivingelements, said fifth NPN transistor having a similar temperaturecharacteristic to that of said fourth NPN transistor; a variableresistor having one terminal connected to an emitter of said fifth NPNtransistor and connected to the emitter of the NPN transistor of thethird transistor circuit of said constant current generating circuit andanother terminal is connected to said negative terminal, and theresistance of said variable resistor varying in accordance with thediaphragm aperture and film speed settings.
 3. An electrical exposurecontrol device for a single lens reflex camera as in claim 1, whereinsaid timing circuit includes a first capacitor connected to thecollector of said fourth NPN transistor, and a second capacitor inparallel with said fourth NPN transistor and said first capacitor, saidsecond capacitor and said series connected first capacitor and saidfourth NPN transistor being selectively connected through a firstchange-over switch to a second variable resistor the resistance of whichvaries according to the diaphragm aperture and film speed settings. 4.An electrical exposure control device for a single lens reflex camera asin claim 3, further comprising: a second change-over switch forestablishing either automatic exposure control or manual exposurecontrol; a level detection circuit connected through said secondchange-over switch to one terminal of either of said first capacitor orsaid second capacitor for detecting the voltage of either of said firstand second capacitors, to thereby generate a signal when said voltagereaches a predetermined level; and an electromagnet for terminatingexposure and actuated by said level detection circuit.
 5. An electricalexposure control device for a single lens reflex camera as in claim 4,wherein said means for storing includes a capacitor for storing theoutput voltage of said compression circuit and a fixed resistorconnected to said capacitor and said negative terminal to therebycompensate for a voltage drop of said power source which results fromthe current flow to said electromagnet.
 6. An electrical exposurecontrol device for a single lens reflex camera as in claim 5, furthercomprising a delay circuit connected between an output terminal of saidlevel detection circuit and an input terminal of said operating circuitto compensate for the time interval representing the OVERLAP between atrailing edge of a first curtain and a leading edge of a second curtainat the cocked position thereof.
 7. An electrical exposure control devicefor a single lens reflex camera as in claim 3, further comprising: afourth transistor circuit connected equivalently to said firsttransistor circuit in said constant current generating circuit andconsisting of a PNP transistor having a base connected to the base ofthe PNP transistor of said first transistor circuit and an emitterconnected through a resistor to said positive terminal, and an NPNtransistor; and a meter circuit to which the output voltage of saidcompression circuit is fed and to which the emitter current of said NPNtransistor of said fourth transistor circuit is fed.